High Performance Air Systems

High Performance Air Systems (HPAS) demand modern design approaches with leading-edge products and technology in order to optimize comfort, while greatly reducing energy consumption. A comfortable environment is achieved by providing the smallest possible zones of temperature control, allowing for individual temperature distribution and better air movement. Reduction in energy is realized by designing a low-pressure HVAC system that allows for maximum turndown, while maintaining temperatures.

Acutherm’s VAV diffusers are available in thermal actuation, Therma-Fuser Series (stand-alone, no wires); as well as, electric actuation, Advantage Series (stand-alone, motorized), and Interoperable Series (BACnet) which can be configured to achieve a High Performance Air Systems design approach. The following five system product configurations are the most commonly used.

Supply Air Temperature

The Therma-Fuser diffuser is a simple stand-alone device that provides VAV control when supplied with air in a suitable range of temperature and pressure.

When supplied with cool air, the Therma-Fuser dampers modulate open on a rise in room temperature. When supplied with warm air, the dampers open on a room temperature drop.

Cool air should be a constant temperature not less than 50°F/10°C for standard models (40°F/4.5°C for model LT-HC) and warm air at a constant temperature of not more than 120°F/49°C. The constant discharge velocity of Therma-Fuser diffusers at varying air flow provides good room circulation which reduces stratification. Keeping heating supply air temperatures as low as possible will further reduce room air stratification to a negligible level.

Changeover from cooling to heating occurs as the supply air rises from 76°F/24.5°C to 80°F/26.5°C and change back from heating to cooling occurs when the supply air drops from 72°F/22°C to 68°F/20°C.

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Static Pressure

Noise level at any air diffuser is influenced by air velocity through the diffuser which is, in turn, a function of static pressure in the neck of the diffuser. Noise level at a Therma-Fuser diffuser will not increase as it closes if the static pressure in the neck is held constant. As static pressure in the neck increases, sound level increases.

Static pressure at the inlet of the Therma-Fuser diffuser must be high enough (0.05”wg /12 Pa or more) to obtain the required air volume for room air induction. Static pressure above 0.25”wg/62 Pa may cause the sound level to become noticeable (NC35). When the static pressure is held constant, the sound level will decrease as the Therma-Fuser dampers close.

Location and Sizing

Because Therma-Fuser diffusers control room temperature by sensing room air induced up the center of the space, care should be taken not to disturb room air induction and entrainment. For example, location next to walls or dropped lights results in the reflection of primary air back at the Therma-Fuser diffuser. Relocate either the Therma-Fuser diffuser or the light.

Therma-Fuser diffusers are sized for the design or maximum air volume required using the published performance guide. Use a larger inlet size at a lower static pressure when lower sound or NC levels are required and when Therma-Fuser diffusers are at the end of a duct run where less static pressure is available.

Therma-Fuser diffusers may be oversized. They will simply turndown air flow to match the space load. Unlike fixed diffusers used with VAV boxes, Therma-Fuser diffusers won’t dump when turned down.

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Duct Design

The goal of a high performance duct design is to design for lowest possible static pressure while providing ample air to each space.

Duct design example: if Therma-Fuser diffusers are to deliver nominal volume at inlet SP of 0.15”wg / 37Pa and if a maximum SP of 0.25”wg / 62Pa is to be held for quiet operation, size the duct for a maximum pressure drop of 0.1”wg /25 Pa between the first and last takeoff.

Manual balancing dampers should be used at the takeoff for each diffuser. Manual balancing dampers may not be required with ducts designed to Acutherm specifications.